Today, lasers are used almost exclusively as illuminating sources in modern two-beam interferometers such as the interferometers of the Fizeau, Twyman-Green or Mach-Zehnder types. This affords the advantage that a high beam flux can be obtained in a very small spatial angle and simultaneously in a very small wavelength range. For the construction and use of the interferometers, the advantage is afforded that interferences occur which are rich in contrast even when the optical paths of the test wave and reference wave are of respectively different lengths. This is a consequence of the fact that laser light of this kind has a very high spatial coherence because of the small spatial angle and has a high temporal coherence because of tile narrow spectral bandwidth.
Because of the high spatial and temporal coherence of the laser light source, the undesirable side effect results that unwanted light in the interferometer (such as scattered light because of residual roughness of the lenses or of the beam splitters) reaches the camera in addition to the two superposed primary waves (test wave and reference wave). The unwanted light is coherently superposed on the primary interference pattern and therefore the interferogram is falsified. This superposition is known as coherent noise.
In interferometry, the task is present to determine the phase difference .phi.(x, y) between the test wave and the reference wave with high spatial resolution as well as with high resolution as to magnitude. The phase differences as a function of the position coordinate or the position coordinates are identified as phase function. The coherent superposed unwanted light leads then to a roughening the phase function .phi.(x, y). Especially high- and mid-frequency spatial frequency components .phi.(x, y) develop with respect to the phase function .phi.(x, y). In the case of a smooth test surface, which should lean to a smooth test wave and therefore also to a smooth function .phi.(x, y), a measured phase function .phi..sub.m (x y) results which shows disturbances .delta.(x y) having short periods.
Published German patent application 3,936,118 discloses a Mireau interferometer wherein a rotating ground glass screen is mounted between the laser and the interferometer input. Each scattering element of the ground glass screen scatters the light in another spatial direction and thereby defines a secondary light source. The light collimated behind the ground glass screen by a lens then has a low spatial coherence since it arises from the incoherent superposition of the light of the many secondary light sources.
Because of the low spatial coherence, the false light then leads to a more or less uniform light distribution which is superposed onto the interferogram made up of the test light wave and the reference light wave. However, it is disadvantageous that, because of the low spatial coherence, also the interference contrast becomes less in the interferogram. This reduction of the interference contrast is that much greater the greater the optical path difference is between the test wave and the reference wave. However, for low interference contrast, the precision with which the phase difference .phi.(x, y) can be determined is less. Accordingly, a rotating ground glass disc of the kind described is not a satisfactory solution in interferometers having different optical path lengths in the reference and measuring beam path such as with a Fizeau interferometer.
U.S. Pat. No. 3,867,009 discloses a holographic microscope having a beam deflection device mounted behind the laser. During the illumination of the photographic film, the direction of the light beams in the reference and in the measuring beam path is varied. In this way, the speckles caused by coherent noise are moved in the hologram plane relative to the image of the object. The interfering light does have high spatial coherence at each time point. However, the variation of the direction of incidence of the light rays on the one hand and the temporal integration which the photographic film carries out on the other hand, characterizes the hologram as a temporally-incoherent sum of spatially coherent rays of varying direction of incidence. Although the coherent noise is suppressed by the temporally-incoherent summation, the interference contrast is however reduced simultaneously if reference beam path and measuring beam path do not have the same optical path lengths which is the case here.
U.S. Pat. No. 4,768,881 discloses a method by means of which the phase function of an individual multiple-fringe interferogram can be computed by means of a Fourier transformation. In addition, the selection of suitable filter functions is suggested for the Fourier transformation in order to suppress during the evaluation those mean or high frequency spatial frequency components which are caused by the coherent noise. The spatial frequency filtering however has the disadvantage that the signal components in the filtered spatial frequency range which are caused by the test object are suppressed to the same extent.
The paper of F. M. Kuchel entitled "Workshop on Optical Fabrication and Testing" published by the Optical Society of America, October 1986, discloses that, for the interferometric testing of mirrors, several measurements are averaged and the test object is rotated about the perpendicular to its surface between the measurements. In this way, the influences of gravitation are eliminated from the averaged measuring result. However, the problem of coherent scattering is not touched in this paper and no suggestion can be derived as to the spatial coherence of the measurement light.